1. Field of the Invention
The present invention relates to a graphite clay composite material, a process for producing the same, a gasket or packing comprising the composite material, and a clay dispersion used for the composite material. More particularly, the graphite clay has essential properties to be used for the gasket or packing such as; heat resistance, durability, barrier (sealing) property or corrosion resistance against a fluid such as a gas or liquid, and furthermore, heat-releasing property, electromagnetic wave shielding property and mechanical strength and the like.
2. Description of the Related Art
In sealing materials such as a joint sheet, gasket, packing and the like, which were used in a connection part of pipes and the like in power plants and chemical plants, asbestos products have been used, neverthless asbestos causes serious health damages. Therefore it is urgent and important developing an alternative materials of asbestos. It is required for the asbestos-replacing materials to have not only high heat resistance and durability but also airtightness and bendability, and furthermore, applicability in wide temperature ranges as well.
Meanwhile, in the field of electronic devices such as computer, CPU, power transistor and the like, a heat releasing sheet for rapidly releasing the heat generated from devices or components, or a shielding sheet for shielding electromagnetic wave from outside are essential components in order to avoid troubles caused by heat and electromagnetic wave. Therefore, materials having superior heat-releasing or electromagnetic wave shielding property are required.
The present invention aims to improve the above situations. Also, the present invention is a new non-asbestos material that replaces asbestos, and has superior heat resistance, durability, sealing property, oxygen resistance, and furthermore, bendability as well. The present invention relates to preferably used for a joint sheet, gasket, packing and the like. The present invention also has heat releasing and electromagnetic wave shielding properties and provides a new material and new technology that are applied to a heat-releasing sheet and an electromagnetic wave shielding sheet.
Currently, commercially available joint sheet, gasket and packing to replace those based on asbestos are based on an organic polymeric material such as rubber, PTFE, aramid and the like, obtained by processing natural or synthetic mineral such as mica, vermiculite and the like into a sheet-like form, made from a metal sheet or a expanded graphite sheet in which a graphite is highly compressed and the like.
However, the product based on an organic polymeric material does not have enough gas barrier property and sealing property while it has a superior bendability. Besides, the product has a problem in heat resistance that the operating temperature is generally up to about 250° C.
In contrast, the mineral product has a high heat resistance and it may be used as a gas seal component such as a gland packing and the like. However, the product is unable to completely block a pass where a small gas molecule flows because of the difficulty in achieving a high density formation. This means gas seal property in the product of mineral is not very high.
The processed product of sheet metal has a superior sealing property and heat resistance. It can also be used in a low temperature range up to 0° C. so that it can be widely applied in several parts like a connection part and the like. However, a metal seal requires a mechanism that tightens the seal and a contact surface may be cracked when the seal is tightened, which may cause a leakage. There are also some problems in the handling. For example, when heating and cooling, the metal seal cannot accommodate the volume change of a peripheral component so that a gap is generated. This results in the leakage or electronic insulation failure.
In contrast, the expanded graphite has a high electrical conductivity as well as a superior alkali resistance and water resistance. The expanded graphite sheet is widely used as a sealing material such as a gasket or packing and the like because of its superiority in heat resistance, flexibility, compression recovery and the like (e.g. Japanese patent publication Tokkai 2005-201363). It is also used as a heat releasing sheet and electromagnetic wave shielding sheet (e.g. Japanese patent publication Tokkai 2000-91453, PCT publication WO99/10598). However it is poor in a mechanical strength such as bending strength, tension strength and the like.
Also, the sealing property of the expanded graphite sheet is not sufficient for sealing gas and organic vapor while it has high heat resistance (gas: about 400° C., liquid: about 600° C.). For example, there is a problem that the present expanded graphite gasket is not applicable for the component used under presence of high temperature oxygen, because it starts oxidation at 500° C. and the expanded graphite sublimates at 600° C. in the presence of oxygen, and the sealing property is lost. Further, there are other problems in handling the present expanded graphite gasket such as; inapplicability to a part where insulation property is required due to its high electrical conductivity, vulnerability to electrolytic corrosion, sensitiveness to strong acid and so on. Moreover, the graphite powder is contaminated in the fluid inside a pipe as an impurity or contaminant, because the graphite sticks to a flange material and the graphite powder easily falls in the fluid.
These days, the expanded graphite is composited with the other material to solve the above-described problems of the expanded graphite sheet. One example is a composite material laminating the expanded graphite sheet with a metal material to reinforce the mechanical strength of the expanded graphite (e.g. Japanese patent publication Tokkai H11-351400). However, such a composite material does not improve the corrosion resistance although it increases the mechanical strength. Even more, no insulation property is expected. The problems of the present expanded graphite composite material are still remained, or is only partly improved and patchworked for its solution. Thus the usage and application of the conventional graphite composite material are limited.
Considering the above situation, the present inventors searched widely substances and materials. As a result, the present inventors found a composition of the clay and graphite improved or newly obtained a mechanical strength, heat resistance, corrosion resistance, gas barrier property, acid resistance, insulation property and the like as well as keeping various superior properties the graphite originally had such as; flexibility, heat conductivity, electrical conductivity, electromagnetic wave shielding property, alkali resistance, water resistance and the like. That is, the present invention provides a graphite clay composite material, process for producing the same, a gasket or packing comprising the composite material, and a clay dispersion used for the composite material. The above-described graphite clay composite can be applied to a wide variety of fields and has a high versatility. The wide variety of fields are, for example, a sealing material such as; joint sheet, gasket, packing and the like used for a connection part of pipes and the like in power plants and chemical plants, heat-release sheet, electromagnetic wave shielding material, sound-proof sheet and the like.